Hitherto, antioxidants have been extensively used in various applications such as plastic products, rubber products, petroleum products (such as lubricating oils) and food. The antioxidants are mainly used for the purpose of inhibiting these products from suffering from undesirable change due to oxygen.
There are conventionally known various kinds of antioxidants. Among these antioxidants, radical chain inhibitors (primary antioxidants) serve for capturing radicals produced upon autoxidation, inhibiting production of the radicals and cutting the radical chain, and are mainly made of hindered phenol-based compounds such as typically 2,6-di-tert-butyl-p-cresol or amine-based compounds such as typically N,N′-diphenyl-p-phenylenediamine. Also, among the antioxidants, peroxide decomposing agents (secondary antioxidants) serve for decomposing peroxides produced upon autoxidation into inert compounds to thereby prevent contribution of the peroxides to chain reaction, and are most frequently made of sulfur-based or phosphorus-based compounds. It is known that when the peroxide decomposing agents are used in combination with the above radical chain inhibitors, a good synergistic effect can be attained.
In the primary antioxidants, in general, the amine-based compounds exhibit a higher oxidation-inhibiting property as compared to the phenol-based compounds. However, since oxidation products produced using the amine-based compounds tend to have a deep color, the use of the amine-based compounds must be limited to applications such as rubber products in which coloration of products causes no significant problems.
As typical amine-based antioxidants used for production of rubber products, there are known N,N′-diphenyl-p-phenylenediamine, N-isopropyl-N′-phenyl-p-phenylenediamine (so-called “3C”), N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (so-called “6C”), etc. In addition, as the amine-based antioxidants, there are also known high-molecular diamines produced from a p-aminophenol derivative and aniline, etc. (JP 53-103429A).
On the other hand, as the typical phenol-based antioxidants, there are known 2,6-di-tert-butyl-p-cresol as a mononuclear compound (so-called “BHT”), 2,2′-methylenebis(4-methyl-6-tert-butylphenol) as a binuclear compound (so-called “2246”), tetrakis[methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]methane as a tetranuclear compound (tradename “Irganox 1010” available from Ciba Specialty Co., Ltd.), etc.
These antioxidants have been required to exhibit a high oxidation inhibiting property even when they are added in a small amount. Further, high-molecular antioxidants are more excellent in view of volatility, migration, extractability, etc., than low-molecular antioxidants. Therefore, in particular in the application field of plastic products, the high-molecular antioxidants tend to be more frequently used.
Meanwhile, aromatic binuclear compounds in which a hydroxyl group and an amino group are respectively bonded to adjacent positions of an aromatic ring thereof have been positively studied as raw materials for polybenzoxazole having excellent heat resistance and electrical insulating property. However, at present, there is almost unknown an oxidation inhibiting property of the aromatic binuclear compounds.